1. Field of the Invention
The present invention relates to a process for producing a hydrogen-containing gas. More particularly, the present invention pertains to a process for producing a reformed gas composed principally of hydrogen which comprises reacting methanol, steam and oxygen through an auto thermal reaction.
2. Description of the Related Arts
In recent years, attention has been paid to hydrogen which takes the place of fossil fuels. Hydrogen is expected as a prospective clean energy in future, since it is burnt into water alone without exhausting carbon dioxide which is the primary contributor to global warming or a harmful nitrogen oxide. It has hitherto been well known that methanol is reformed into a hydrogen-containing gas in the presence of a catalyst in a comparatively easy manner. In addition, methanol, which is reformed by reaction with steam (steam-reforming reaction) into a hydrogen-containing gas having a high concentration of hydrogen, has recently become a center of attraction as a simple convenient supply source of hydrogen which is expected to expand its demand in the near future.
On the other hand, during enhanced concern about global environment problem, attention has been given to a fuel cell as a clean power generation system which suppresses the exhaust of carbon dioxide being the primary contributor to global warming and which never exhausts a nitrogen oxide bringing about air pollution. The fuel cell is expected to be utilized not only as a power source for a moving object such as an automobile and marine vessel but also as a private power generation source for a factory, building, collective housing and the like, and an uninterruptible power source for hospitals dealing with medical machinery and equipment. In particular in the field of automobiles, development is set forward in a positive manner on an automobile which is driven by a methanol-reformed fuel. Since a fuel cell which is mounted on an automobile is called upon to be miniatured and simplified in its structure, development is set forward on a auto thermal reformer in which reaction heat necessary for steam-reforming reaction is supplied by the heat of combustion through the introduction of air into the reactor.
In a reactor of the auto thermal reaction, where methanol, steam and oxygen are reacted, part of methanol is burnt, so that the portion where oxidative reaction takes place is brought to a high temperature as compared with the steam-reforming reaction, and thereby a catalyst having high heat resistance is required.
In vehicle mounting application in which limitations are put on a mounting capacity and the like, a reforming reactor is called upon to be miniaturized, thus requiring a highly active and highly durable catalyst. Further in the case of a solid polymer electroyte fuel cell, since carbon monoxide in a reformed gas is adsorbed onto platinum of an electrode catalyst, thereby greatly deteriorating the function of the cell, the concentration of carbon monoxide in the reformed gas is preferably as low as possible.
The principal reaction in the steam-reforming reaction of methanol is represented by the chemical equations (i) and (ii):CH3OH→CO+2H2−90.7 kJ/mol  (i)CH3OH+H2O→CO2+3H2−49.5 kJ/mol  (ii)
In order to efficiently produce hydrogen from methanol, the reaction in the chemical equation (ii) is more advantageous than the reaction in (i) in terms of much production amount of hydrogen, less endothermic reaction heat and non-formation of carbon monoxide.
There is proposed as a methanol reforming catalyst, a catalyst in which any of a variety of catalytic substrates is supported on a carrier such as alumina and silica.
For instance, there are proposed the catalyst comprising at least one metal which is selected from the group consisting of platinum and palladium and which is supported on a carrier wherein alumina is coated in advance with the oxide of an alkaline substance in Japanese Patent Application Laid-Open No. 68140/1982 (Showa 57); the methanol reforming catalyst which comprises nickel supported on a carrier comprising the oxide or hydroxide of at least one metal selected from the group consisting of copper, zinc and chromium in Japanese Patent Application Laid-Open No. 174138/1982 (Showa 57); and the methanol reforming catalyst comprising the oxide or hydroxide of at least one metal selected from the group consisting of copper, zinc and chromium and the oxide or hydroxide of nickel in Japanese Patent Application Laid-Open No. 174139/1982 (Showa 57). The catalysts proposed in the above-cited Japanese Patent Applications Laid-Open Nos. 68140/1982 (Showa 57), 174138/1982 (Showa 57) and 174139/1982 (Showa 57), respectively involve the problems of insufficient catalytic activity at a low temperature, liability to carbon deposition resulting in limited service life and the like problems.
In addition, there is proposed the alumina-based methanol decomposing catalyst comprising nickel and potassium incorporated in alumina in Japanese Patent Application Laid-Open No. 144031/1982 (Showa 57), but the catalyst involves the problems that a conversion at a reaction temperature of 350° C. is as low as, for instance, 52% and that for the purpose of increasing the conversion up to 75 to 91%, the catalyst must be subjected to a pretreatment at 500° C. in a stream of argon.
There is proposed the methanol reforming catalyst comprising at least one metal which is selected from the group consisting of platinum and palladium, and which is supported on a carrier wherein alumina is coated in advance with the oxide of an alkali metal in Japanese Patent Application Laid-Open No. 199043/1984 (Showa 59). The catalysts in Japanese Patent Applications Laid-Open Nos. 199043/1984 (Showa 59) and 68140/1982 (Showa 57) as mentioned hereinbefore suffer from an industrial disadvantage that they must be subjected to coating treatment in advance with the oxide of an alkaline substance (oxide of an alkali metal).
With regard to the above-cited Japanese Patent Applications Laid-Open Nos. 68140/1982 (Showa 57), 174138/1982 (Showa 57), 174139/1982 (Showa 57), 144031/1982 (Showa 57) and 199043/1984 (Showa 59), respectively, the reaction represented by the chemical equation (i) is the principal reaction, and carbon monoxide with a concentration of about 30% is contained in the reformed gas. Accordingly, it is necessary to produce hydrogen and carbon dioxide by means of water-gas shift reaction and separate the hydrogen. Further, the methanol decomposing reaction belongs to endothermic reaction, to which heat must be supplied from the outside of the reaction system, whereby the related equipment becomes intricate and troublesome.
As the methanol-reforming catalyst directed principally to the reaction represented by the chemical equation (ii), there has heretofore been proposed a copper-zinc base catalyst which is active at a low temperature and is high in selectivity to the reaction represented by the chemical equation (ii). However, the above-mentioned copper-zinc base catalyst suffers from such disadvantages that it is poor in heat resistance, and when put into a continuous operation for a long time, the catalytic activity and selectivity thereof are rapidly and continuously deteriorated.
On the contrary, there are proposed the catalyst comprising palladium and zinc oxide as the catalyst having excellent heat resistance and relatively high activity and durability in Japanese Patent Application Laid-Open No. 49930/1993 (Heisei 5); the catalyst comprising palladium or platinum each supported on a zinc oxide carrier in Japanese Patent Application Laid-Open No. 25662/2001 (Heisei 13); the catalyst which comprises a noble metal such as platinum, palladium, rhodium or indium each supported on a carrier comprising a basic metal oxide such as selenium dioxide or zirconium dioxide, and which also comprises an alkali metal or an alkaline earth metal supported thereon in Japanese Patent Application Laid-Open No. 246106/2000 (Heisei 12); the catalyst which comprises a noble metal (platinum), rhenium and an element belonging to the group 2B or 3B each supported on a carrier comprising a metal oxide such as aluminum oxide, cerium dioxide or zirconium dioxide in Japanese Patent Application Laid-Open No. 342968/2000 (Heisei 12); and the catalyst comprising palladium-gallium oxide, palladium-indium oxide, platinum-zinc oxide, platinum-gallium oxide or platinum-indium oxide in SHOKUBAI (Catalyst as a periodical) vol.42 (2000) pp. 212 to 217.
However, the above-described catalysts, which is directed principally to the reaction represented by the chemical equation (ii), suffer from drawbacks in that when used in auto thermal reaction by introducing air along with steam, carbon monoxide is formed in high concentration as compared with the copper-zinc base catalyst, and the selectivity to the reaction represented by the chemical equation (ii) is lowered during a long time service, whereby the carbon monoxide concentration is further increased.
Moreover, an evaluation is made of the catalyst through auto thermal reaction by the use of the catalyst comprising copper-zinc incorporated with palladium as a methanol-reforming catalyst for lowering the concentration of carbon monoxide in the reformed gas in Japanese Patent Application Laid-Open No. 185192/2001 (Heisei 13). The catalyst has excellent heat resistance and high activity, but is expected to be more improved.
That is to say, in the auto thermal reactor where methanol, steam and oxygen are reacted with one another, as part of methanol is oxidized into hydrogen and carbon dioxide, the portion where oxidative reaction takes place is brought to a high temperature as compared with the steam-reforming reaction, and thereby a catalyst having high heat resistance is required.
In vehicle mounting application wherein limitations are put on a mounting capacity, a reforming reactor needs to be miniatured, and thereby a catalyst having a high catalytic activity is required. In particular for fuel cell application, it is required that the reformed gas have a low carbon monoxide concentration, and that the selectivity to the reaction represented by the chemical equation (ii) is enhanced.
Under such circumstances, the object of the present invention is to provide a process for producing a gas comprising hydrogen as the principal ingredient in an industrially advantageous manner by means of an auto thermal reactor where methanol, steam and oxygen are reacted with one another, by developing a methanol reforming catalyst having a high activity, high durability and high selectivity to steam-reforming reaction.
Other objects of the present invention will become obvious from the text of this specification hereinafter disclosed.